: The long-term goal of this project is to elucidate the mechanisms that control cell polarity using the C. elegans 1-cell embryo (zygote) as model system. C. elegans offers two key advantages for this work: 1) the ability to visualize specific protein components of the polarity machinery in live cells, and 2) the ability to rapidly identify genes required for polarity using both forward and reverse genetic approaches. Establishment of the anterior-posterior (AlP) body axis of C. elegans depends on polarization of the zygote shortly after fertilization. The relevance of this system to other polarized cells was firmly established last year, when homologues of the core components of the C. elegans A/P polarity machinery were discovered in Drosophila and mammals, and shown to have similar polarity functions. Recently, we demonstrated that A/P polarity in the C. elegans zygote is triggered by microtubules emanating from the sperm asters; earlier studies also indicated a role for the actin cytoskeleton. The goal of this proposal is to identify the molecular mechanisms that link the core A/P polarity machinery to the microtubule and actin cytoskeletons. To this end, we have developed a method to monitor the establishment of A/P polarity directly in live zygotes. We will use this method to characterize the dynamics of AfP polarity, and to identify new genes involved in this process. Our screening strategy will use a functional genomics approach as well as standard genetic methods. In a pilot screen, we identified a new gene, porn-I, required for maintenance of the polarity axis during mitosis and alignment of the spindle along that axis. POM-1 is related to Pomip, a protein kinase required for polarized growth and cell division in S. pombe. Our initial findings with porn-i reveal the existence of a previously unrecognized link between spindle alignment and the A/P polarity machinery in the zygote. Cell polarity is at the core of many essential processes in animals including asymmetric cell divisions, the functioning of polarized cell types (e.g. neurons and epithelial cells), and inhibition of unregulated cell proliferation. We anticipate that our studies will provide significant insights into these processes by taking advantage of the experimental attributes of a simple model system.